Base layer, polishing pad including the same and polishing method

ABSTRACT

A polishing pad including a polishing layer and a base layer is provided. Disposed under the polishing layer, the base layer includes a porous inner layer and at least one surface layer. The porous inner layer has an upper surface and a lower surface. The surface layer is disposed on at least one of the upper surface and the lower surface of the porous inner layer. The surface layer has a pore ratio no larger than 0.3%, or is completely non-porous.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of Taiwan application serial no. 99114084, filed on May 3, 2010. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a base layer, a polishing pad including the same, and a polishing method. More particularly, the invention relates to a base layer having better shear stress resistance with an adhesive layer, a polishing pad including the base layer, and a polishing method.

2. Description of Related Art

With the progress of the industry, planarization processes are often adopted for manufacturing various devices. A chemical mechanical polishing(CMP) process is often used in the planarization process in the industry. General speaking, the chemical mechanical polishing process supplies slurry having a chemical on the polishing pad, applies a pressure on the substrate to be polished to press it on the polishing pad, and provides a relative motion between the substrate and the polishing pad. Through the mechanical friction generated by the relative motion and the chemical effects of the slurry, a portion of the surface layer of the substrate is removed to make the surface flat and smooth so as to achieve planarization.

Conventional polishing pads used in semiconductor wafers have multiple-layer structures, and each structure normally includes a polishing layer, an adhesive layer, and a base layer. The polishing layer has a polishing surface capable of contacting the object to be polished directly. The polishing layer can have microporous, grooves, and/or through holes formed thereon. The base layer is adhered under the polishing layer and fixed on a polishing machine. The base layer is a porous structure with numerous pores of various sizes. A ratio of a pore area occupying a total area is referred as a pore ratio. The pore ratio is basically larger than 20% and can even reach 60% or more. The adhesive layer mainly provides an adhesive force for adhering the polishing layer and the base layer tightly.

In the polishing process, another adhesive layer is first used to adhere the other surface of the base layer (the surface not contacting the foregoing adhesive layer) on the platen of the polishing machine. Afterwards, the object (i.e. wafer or substrate) to be polished is polished utilizing the polishing pad.

Nevertheless, since the base layer is a porous structure with numerous pores of various sizes, the contact surface adhered to the adhesive layer also includes a plurality of pores. In the polishing process, these pores are deformed due to the shear force and therefore compress the air. The compressed air consequently extrudes the adhesive layer originally adhered to the pores. Micro-protrusions are thus generated on the surface of the base layer contacting the adhesive layer. With the increase of polishing time, the surface contacting the base layer and the adhesive layer gradually becomes uneven under the continuous shear force, which eventually leads to degraded polishing quality and erroneous fabrication.

SUMMARY OF THE INVENTION

Accordingly, the invention relates to a base layer having better shear stress resistance with the adhesive layer.

The invention relates to a polishing pad including a polishing layer and a base layer. The base layer is disposed under the polishing layer and includes a porous inner layer and at least one surface layer. The porous inner layer has an upper surface and a lower surface. The surface layer can have few pores (with the pore ratio no larger than 0.3%) or is completely non-porous (i.e. does not have any pores). Moreover, the surface layer is disposed on at least one of the upper surface and the lower surface of the porous inner layer.

The invention also relates to a base layer adapted for underlying a polishing layer of a polishing pad. The base layer includes a porous inner layer and a surface layer. The porous inner layer has an upper surface and a lower surface. The surface layer can have few pores (with the pore ratio no larger than 0.3%) or is completely non-porous (i.e. does not have any pores). Moreover, the surface layer is disposed on at least one of the upper surface and the lower surface of the porous inner layer.

The invention further relates to a polishing method adapted for polishing a substrate. The polishing method includes the following steps. A polishing pad is first provided. A pressure is applied on the substrate to press the substrate on the polishing pad. A relative motion is then provided between the substrate and the polishing pad. Herein, the polishing pad includes a polishing layer and a base layer. The base layer is disposed under the polishing layer and includes a porous inner layer and at least one surface layer. The porous inner layer has an upper surface and a lower surface. The surface layer can have few pores (with the pore ratio no larger than 0.3%) or is completely non-porous (i.e. does not have any pores). The surface layer is disposed on at least one of the upper surface and the lower surface of the porous inner layer.

In light of the foregoing, since the surface layer in the base layer provided in the invention has the pore ratio no larger than 0.3% or is completely non-porous (i.e. does not have any pores), when the surface layer is non-porous, a surface of the surface layer contacting the adhesive layer is also non-porous (i.e. does not have any pores). Consequently, the conventional uneven surface of the base layer contacting the adhesive layer caused by the porous structure of the base layer does not result. When the surface layer includes few pores (with the pore ratio no larger than 0.3%), the number of pores accumulated on the surface of the base layer contacting the adhesive layer is few. Although the pores are also deformed due to the shear force and therefore compress the air, since the few number of pores are not sufficient for compressing the air to extrude the adhesive layer originally adhered to the pores, the surface of the base layer contacting the adhesive layer does not become uneven and the polishing quality thereof is not affected. Therefore, the base layer disclosed in the invention is tightly adhered to the adhesive layer through the surface layer such that the base layer has better shear stress resistance with the adhesive layer. In addition, in the polishing pad disclosed in the invention, since the base layer in the polishing pad has a surface layer with a pore ratio no larger than 0.3% or has a non-porous surface layer, the chance of the adhesive layer peeling off is therefore reduced.

In order to make the aforementioned and other features and advantages of the invention more comprehensible, several embodiments accompanied with figures are described in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

FIG. 1 illustrates a cross-sectional view of a polishing pad according to a first embodiment of the invention.

FIG. 2 illustrates a cross-sectional view of a polishing pad according to a second embodiment of the invention.

FIG. 3 illustrates a cross-sectional view of a polishing pad according to a third embodiment of the invention.

FIG. 4 shows a scanning electron microscope (SEM) diagram of a base layer according to an embodiment of the invention.

FIG. 5 illustrates a flowchart of a polishing method according to an embodiment of the invention.

DESCRIPTION OF EMBODIMENTS

FIG. 1 illustrates a cross-sectional view of a polishing pad according to a first embodiment of the invention. FIG. 2 illustrates a cross-sectional view of a polishing pad according to a second embodiment of the invention. FIG. 3 illustrates a cross-sectional view of a polishing pad according to a third embodiment of the invention. FIG. 4 shows a scanning electron microscope (SEM) diagram of a base layer according to an embodiment of the invention.

Referring to FIGS. 1-3 simultaneously, a polishing pad 100 disclosed in the invention includes a polishing layer 102 and a base layer 104. Here, the polishing layer 102 is manufactured using a polymer material. The polymer material includes, for example, polyurethane (PU). The polishing layer 102 has a polishing surface capable of contacting an object (i.e. wafer or substrate) to be polished directly. Moreover, the polishing layer 102 can have microporous, grooves, and/or through holes formed thereon.

The base layer 104 is adapted for underlying the polishing layer 102 in the polishing pad 100. The base layer 104 is adhered under the polishing layer 102 and includes a porous inner layer 106 and at least one surface layer 108. Herein, the surface layer 108 is a non-porous structure, which has few pores or is completely non-porous (i.e. does not have any pores). Thus, the surface layer 108 is also referred as the non-porous surface layer 108.

The porous inner layer 106 and the non-porous surface layer 108 are manufactured by using the same material, for instance. The porous inner layer 106 and the non-porous surface layer 108 have identical chemical structures. The material is low density polyethylene or a mixture of low density polyethylene and ethylene vinyl acetate, for example. The porous inner layer 106 and the non-porous surface layer 108 can concurrently be made from the same foaming process. The porous inner layer 106 and the non-porous surface layer 108 can be formed simultaneously with the same material by controlling the foaming time and the foaming temperature. In other words, the porous inner layer 106 and the non-porous surface layer 108 are manufactured as an integrative unit instead of being formed individually and then contacted (or adhered) with each other. Moreover, the porous inner layer 106 and the non-porous surface layer 108 do not have an obvious layering relationship. Specifically, although the porous inner layer 106 and the non-porous surface layer 108 are both referred as “layer” in the invention, from the SEM diagram shown in FIG. 4, it is clear that these two layers (106, 108) represent a self-formed double-layered structure which do not have a distinct joint line.

The porous inner layer 106 has an upper surface 110 and a lower surface 112. Also, the porous inner layer 106 has a plurality of micro pores 114 with various sizes. The pore size distribution of the pores 114 ranges from 10 μm to 400 μm, for example. In one embodiment, the average pore size of the pores 114 ranges from 100 μm to 250 μm.

Herein, the non-porous surface layer 108 optionally has few pores or is completely non-porous (i.e. does not have any pores). In one embodiment, the pore ratio of the non-porous surface layer 108 is no larger than 0.3% (i.e. no larger than 0.2%; no larger than 0.1%; or can even be 0%). It should be noted that the pore ratio illustrated in the invention is defined as the ratio of the pore area occupying the total area of the non-porous surface layer 108. The ratio not larger than 0.3% means the ratio is either smaller or equal to 0.3%. In the present embodiment, although the non-porous surface layer 108 still has a plurality of micro pores, the number of pores is reduced comparing to the porous structure of the conventional base layer (ex. having the pore ratio larger than 20%). Therefore, although the pores are still present, the few pores are not sufficient to accumulate on the surface of the non-porous surface layer 108 contacting the adhesive layer so as to compress air for generating micro-protrusions leading to uneven contact surface and, therefore, poor polishing quality.

Obviously, the manufacturers are capable of determining the pore ratio of the non-porous surface layer 108 through the control of foaming time and foaming temperature. In one embodiment, the pore ratio of the non-porous surface layer 108 is 0%. That is, the non-porous surface layer 108 does not have any pores. Since the surface layer does not have any pores, pores do not accumulate on the surface of the non-porous surface layer 108 contacting the adhesive layer. As a result, the degradation in polishing quality caused by uneven contact surface does not occur.

At the same time, the non-porous surface layer 108 is optionally disposed on at least one of the upper surface 110 and the lower surface 112 of the porous inner layer 106. In details, the non-porous surface layer 108 is merely disposed on the upper surface 110 of the porous inner layer 106, merely disposed on the lower surface 112 of the same, or disposed on the upper surface and lower surface (110, 112) of the porous inner layer 106 simultaneously. Depending on product demands, the manufacturers can adjust the method of fabrication to manufacture the non-porous surface layer 108 disposed on different locations or having different pore ratios.

As shown in FIG. 1, the non-porous surface layer 108 is disposed on the upper surface 110 of the porous inner layer 106. According to this disposition, the non-porous surface layer 108 is sandwiched between the porous inner layer 106 and the polishing layer 102.

As shown in FIG. 2, the non-porous surface layer 108 is disposed on the lower surface 112 of the porous inner layer 106. According to this disposition, the non-porous surface layer 108 is sandwiched between the porous inner layer 106 and a platen 10 of the polishing machine.

As illustrated in FIG. 3, the non-porous surface layer 108 is disposed on the upper surface 110 and the lower surface 112 of the porous inner layer 106 simultaneously. According to this disposition, the non-porous surface layer 108 disposed on the upper surface 110 is sandwiched between the porous inner layer 106 and the polishing layer 102. On the other hand, the non-porous surface layer 108 disposed on the lower surface 112 is sandwiched between the porous inner layer 106 and the platen 10 of the polishing machine.

Here, the non-porous surface layer 108 has a thickness larger than 5 μm, for example. In one embodiment, the thickness of the non-porous surface layer 108 ranges from 8 μm to 35 μm. The thickness of the non-porous surface layer 108 can be manipulated through adjusting parameters in the fabrication of the base layer 104. For instance, the thickness can be manipulated by adjusting the surface temperature of mold used in a batch production or by adjusting the surface temperature of rollers adopted in a continuous production. The surface temperatures of the mold or the rollers are exemplary lower than 30° C., 20° C., 10° C., or even lower than 5° C. Generally, the lower the surface temperature of the mold or the rollers, the thicker the manufactured non-porous surface layer 108 is. In addition, a base layer with a porous surface layer can be first manufactured. Thereafter, the porous surface layer is heated and pressurized for turning the surface layer into a non-porous surface layer. Here, the thickness of the non-porous surface layer is manipulated through the adjustment of temperature and pressure parameters. Additionally, the surface roughness of the non-porous surface layer 108 is smaller than, for instance, 15 μm. For example, the surface roughness of the non-porous surface layer 108 ranges from, for instance, 3 μm to 10 μm. During the fabrication, a surface treatment can be first applied to the non-porous surface layer 108 prior to the adhesion of the non-porous surface layer 108 and the adhesive layer for enhancing a surface adhesive force of the non-porous surface layer 108. Here, the surface treatment is, for instance, a plasma treatment.

In addition, the polishing pad 100 further includes an adhesive layer 116 sandwiched between the polishing layer 102 and the base layer 104 for adhering the polishing layer 102 and the base layer 104 so as to form the polishing pad 100. The adhesive layer 116 is made of pressure sensitive adhesive (PSA), for instance.

As illustrated in FIG. 1, when the non-porous surface layer 108 is disposed on the upper surface 110 of the porous inner layer 106, since the non-porous surface layer 108 has few pores (with the pore ratio no larger than 0.3%) or is completely non-porous (i.e. does not have any pores), the surface of the surface layer contacting the adhesive layer does not have any pores when the surface layer is non-porous. Consequently, the uneven surface of the base layer contacting the adhesive layer caused by the porous structure of the base layer does not result. When the surface layer has few pores (with the pore ratio no larger than 0.3%), the surface contacting the non-porous surface layer 108 (the base layer 104) and the adhesive layer 116 only has few pores. Therefore, when the non-porous surface layer 108 (the base layer 104) is under the shear force during the polishing process, the chance of having the micro-protrusions formed on the surface of the base layer 104 contacting the adhesive layer 116 by the air compression of the deformed pores can be reduced, such that the base layer 104 can be tightly adhered to the adhesive layer 116. As a result, the polishing layer 102 and the base layer 104 remain well-adhered during the polishing process.

As shown in FIG. 2, when the non-porous surface layer 108 is disposed on the lower surface 112 of the porous inner layer 106, a surface of the non-porous surface layer 108 not contacted with the porous inner layer 106 then adheres to one surface of another adhesive layer 118. Moreover, the other surface of the adhesive layer 118 is adhered to the platen 10 of the polishing machine. Similarly, as the non-porous surface layer 108 has few pores (with the pore ratio no larger than 0.3%) or is completely non-porous (i.e. does not have any pores), the surface of the non-porous surface layer 108 (the base layer 104) contacting the adhesive layer 118 only has few pores or does not have any pores. Therefore, when the non-porous surface layer 108 (the base layer 104) is under the shear force during the polishing process, the chance of having the micro-protrusions formed on the contact surface between the base layer 104 and the adhesive layer 118 by the air compression of the deformed pores can be reduced, such that the base layer 104 can be tightly adhered to the adhesive layer 118. As a result, the polishing pad 100 and the polishing machine remain well-adhered during the polishing process.

Referring to FIG. 3, when the upper surface 110 and the lower surface 112 of the porous inner layer 106 are simultaneously disposed with the non-porous surface layer 108, the contact surfaces of the non-porous surface layers 108 (the base layer 104) adhering to the adhesive layers (116, 118) merely include few pores or are completely non-porous (i.e. does not have any pores). Therefore, when the non-porous surface layers 108 (the base layer 104) are under the shear force during the polishing process, the chance of having the micro-protrusions formed on the surfaces of the base layer 104 contacting the adhesive layers 116, 118 by the air compression of the deformed pores can be reduced, such that the base layer 104 can be tightly adhered to the adhesive layers 116, 118. As a result, the polishing layer 102 and the base layer 104 remain well-adhered during the polishing process. The polishing pad 100 and the polishing machine also remain well-adhered in the polishing process.

In the invention, in order to show the effect of the polishing pad provided in the invention, a shear stress resistance experiment is implemented.

EXPERIMENTAL EXAMPLE

The features of the base layer in the polishing pad are illustrated with the following actual experiments. In the experiments, testing methods and sample settings are presented below.

Shear stress resistance tests: measuring the shear stress resistance maintenance time of the base layer using the ASTM D3652 standard test method.

Material of adhesive layer: acrylic-based adhesive.

Sample: polishing pad obtained by adhering the polishing layer and the base layer with the adhesive layer. The difference between Experiment 1 and Comparative Example 1 lies in the different base layers.

Example 1

A base layer having a porous inner layer and a non-porous surface layer.

Comparative Example 1

A base layer having the same material and the pore structure as the base layer in Example 1, but does not include the non-porous surface layer; that is, the surface layer is a porous base layer, for example, a base layer obtained by removing the non-porous surface layer from the base layer in Example 1.

In the following, the experimental results describing the adhesive features between the base layer and the adhesive layer are listed in Table 1 below.

TABLE 1 Example 1 Comparative Example 1 Shear stress resistance 74 14 maintenance time (hr) Generation of micro- No Yes protrusions during actual polishing?

Referring to the experimental results of Example 1 and Comparative Example in Table 1, the base layer having the non-porous surface layer in Example 1 has a shear stress resistance maintenance time of 74 hours, which has an improvement range exceeding 400% comparing to a shear stress resistance maintenance time of 14 hours of the base layer having the porous surface layer in Comparative Example 1. In the polishing process, the polishing pad must not be peeled off under the shear stress effect during the long period of time. Thus, the shear stress resistance maintenance time is an important index for testing the performance of the polishing pad. Accordingly, the presence of the base layer having the non-porous surface layer is essential.

In the shear stress resistance maintenance time test, after the non-porous surface layer 108 of the base layer 104 and the adhesive layer (116 or 118) are adhered, the base layer 104 and the adhesive layer (116 or 118) have a first maintenance time therebetween when a shear stress is applied to the non-porous surface layer 108. When the same shear stress is applied to the base layer, which has the same material and the same pore structure but does not include the non-porous surface layer; that is, when the same shear stress is applied to the base layer having the porous surface layer adhering to the same adhesive layer (i.e. the same shear stress is applied to the porous inner layer 106 which has the non-porous surface layer 108 removed and is adhered to the same adhesive layer), the base layer 104 and the adhesive layer have a second maintenance time therebetween. The first maintenance time is at least 20% longer than the second maintenance time (for example, 50%, 100%, 200%, 300%, 400%, or 500% longer).

In light of the foregoing, since the non-porous surface layer 108 in the base layer 104 has only few pores or is completely non-porous (i.e. does not have any pores), the base layer 104 is capable of adhering to the adhesive layer (116 or 118) tightly through the non-porous surface layer 108, so that the base layer 104 and the adhesive layer have better shear stress resistance.

FIG. 5 illustrates a flowchart of a polishing method according to an embodiment of the invention. FIG. 5 illustrates a flowchart of a polishing method according to an embodiment of the invention. This polishing method is adapted for polishing a substrate.

Referring to FIG. 5, firstly, step S100 is performed, where a polishing pad is provided. The polishing pad is, for example, the polishing pad 100 shown in embodiments of FIGS. 1-3. The polishing pad includes a polishing layer and a base layer. The base layer is adhered under the polishing layer and includes a porous inner layer and a surface layer (non-porous surface layer). Herein, the porous inner layer has an upper surface and a lower surface. The surface (non-porous surface layer) can have few pores (with the pore ratio no larger than 0.3%) or be completely non-porous (with the pore ratio equals to 0%). The surface layer (the non-porous surface layer) is optionally adhered to the upper surface of the porous inner layer, to the lower surface of the porous inner layer, or to both the upper and the lower surfaces of the porous inner layer.

Next, step S102 is carried out, where a pressure is applied to the substrate to press the substrate on the foregoing polishing pad, so that the substrate and the polishing pad contact each other.

Thereafter, step S104 is performed for providing a relative motion between the substrate and the polishing pad, so as to remove a portion of a substrate surface to achieve planarization.

In summary, in one embodiment, the base layer disclosed in the invention has better shear stress resistance with the adhesive layer. In one embodiment, the polishing pad disclosed in the invention reduces the generation of micro-protrusions on the surface of the base layer contacting the adhesive layer.

Although the invention has been described with reference to the above embodiments, it will be apparent to one of the ordinary skill in the art that modifications to the described embodiment may be made without departing from the spirit of the invention. Accordingly, the scope of the invention will be defined by the attached claims not by the above detailed descriptions. 

1. A polishing pad, comprising: a polishing layer; and a base layer, disposed under the polishing layer, and comprising: a porous inner layer having an upper surface and a lower surface; and at least one surface layer having a pore ratio no larger than 0.3% and disposed on at least one of the upper surface and the lower surface of the porous inner layer.
 2. The polishing pad as claimed in claim 1, wherein the surface layer is a non-porous surface layer and has a pore ratio of 0%.
 3. The polishing pad as claimed in claim 2, wherein the porous inner layer and the non-porous surface layer are manufactured integrally using a same material.
 4. The polishing pad as claimed in claim 2, wherein a thickness of the non-porous surface layer is larger than 5 μm.
 5. The polishing pad as claimed in claim 4, wherein the thickness of the non-porous surface layer ranges from 8 μm to 35 μm.
 6. The polishing pad as claimed in claim 2, wherein a surface roughness of the non-porous surface layer is smaller than 15 μm.
 7. The polishing pad as claimed in claim 6, wherein the surface roughness of the non-porous surface layer ranges from 3 μm to 10 μm.
 8. The polishing pad as claimed in claim 2, wherein a pore size distribution of pores in the porous inner layer ranges from 10 μm to 400 μm.
 9. The polishing pad as claimed in claim 8, wherein an average pore size of the pores in the porous inner layer ranges from 100 μm to 250 μm.
 10. The polishing pad as claimed in claim 1, wherein the base layer is manufactured using low density polyethylene or a mixture of low density polyethylene and ethylene vinyl acetate.
 11. The polishing pad as claimed in claim 2 further comprising an adhesive layer at least disposed between the polishing layer and the base layer or between the base layer and a polishing machine.
 12. The polishing pad as claimed in claim 11, wherein a shear stress resistance maintenance time test is performed after the base layer and the adhesive layer are adhered; when a shear stress is applied to the non-porous surface layer, the base layer and the adhesive layer have a first maintenance time therebetween; when the shear stress is applied to the porous inner layer, the base layer and the adhesive layer have a second maintenance time therebetween; the first maintenance time is at least 20% longer than the second maintenance time.
 13. The polishing pad as claimed in claim 2, wherein the non-porous surface layer comprises a non-porous surface layer underwent a plasma treatment.
 14. A base layer adapted for underlying a polishing layer of a polishing pad, the base layer comprising: a porous inner layer having an upper surface and a lower surface; and a surface layer having a pore ratio no larger than 0.3% and disposed on at least one of the upper surface and the lower surface of the porous inner layer.
 15. The base layer as claimed in claim 14, wherein the surface layer is a non-porous surface layer and has a pore ratio of 0%.
 16. The base layer as claimed in claim 15, wherein the porous inner layer and the non-porous surface layer are manufactured integrally using a same material.
 17. The base layer as claimed in claim 15, wherein a thickness of the non-porous surface layer is larger than 5 μm.
 18. The base layer as claimed in claim 17, wherein the thickness of the non-porous surface layer ranges from 8 μm to 35 μm.
 19. The base layer as claimed in claim 15, wherein a surface roughness of the non-porous surface layer is smaller than 15 μm.
 20. The base layer as claimed in claim 19, wherein the surface roughness of the non-porous surface layer ranges from 3 μm to 10 μm.
 21. The base layer as claimed in claim 15, wherein a pore size distribution of pores in the porous inner layer ranges from 10 μm to 400 μm.
 22. The base layer as claimed in claim 21, wherein an average pore size of the pores in the porous inner layer ranges from 100 μm to 250 μm.
 23. The base layer as claimed in claim 14, wherein the base layer is manufactured using low density polyethylene or a mixture of low density polyethylene and ethylene vinyl acetate.
 24. The base layer as claimed in claim 15, wherein the non-porous surface layer comprises a non-porous surface layer underwent a plasma treatment.
 25. A polishing method adapted to polish a substrate, the method comprising: providing a polishing pad; applying a pressure on the substrate to press the substrate on the polishing pad; and providing a relative motion between the substrate and the polishing pad, wherein the polishing pad comprises: a polishing layer; and a base layer, disposed under the polishing layer, and comprising: a porous inner layer having an upper surface and a lower surface; and at least one surface layer having a pore ratio no larger than 0.3% and disposed on at least one of the upper surface and the lower surface of the porous inner layer.
 26. The polishing method as claimed in claim 25, wherein the surface layer is a non-porous surface layer and has a pore ratio of 0%.
 27. The polishing method as claimed in claim 26, wherein the porous inner layer and the non-porous surface layer are manufactured integrally using a same material.
 28. The polishing method as claimed in claim 26, wherein a thickness of the non-porous surface layer is larger than 5 μm.
 29. The polishing method as claimed in claim 26, wherein a surface roughness of the non-porous surface layer is smaller than 15 μm.
 30. The polishing method as claimed in claim 26, wherein a pore size distribution of pores in the porous inner layer ranges from 10 μm to 400 μm.
 31. The polishing method as claimed in claim 25, wherein the base layer is manufactured using low density polyethylene or a mixture of low density polyethylene and ethylene vinyl acetate.
 32. The polishing method as claimed in claim 26, wherein the non-porous surface layer comprises a non-porous surface layer underwent a plasma treatment. 